1. Field of the Invention
This invention relates to an image pickup apparatus having a gradation control function suitable for performing image pickup of a backlighted object.
2. Description of the Related Art
It is inevitable that a wide range of gradations in one picture be produced for picking up by use of a video camera or the like an image of a backlighted object in which illuminance of the backlighting is intense.
An image pickup apparatus provided with a solid image pickup element for use in a video camera or the like does not possess a wide dynamic range in such a degree as to entirely convert such wide gradations to video signals. This causes clipping in the respective higher and lower levels of illuminance, which is referred to as xe2x80x9cwhite compressionxe2x80x9d and xe2x80x9cblack compressionxe2x80x9d respectively in which excessively bright and dark portions cannot be expressed in gradations.
In particular, when image pickup is performed on a backlighted object, the main object becomes darkly shaded in contrast to the bright background, thus causing xe2x80x9cblack compressionxe2x80x9d.
In order to deal with such inconveniences, a BLC (backlight control) method is conventionally employed whereby exposure is increased to perform backlight compensation. The backlight compensation will now be explained in detail.
A description will first be given of the operation of the BLC for use with an exposure control mechanism using an iris with reference to a diagram of a backlight compensation circuit shown in FIG. 22.
An incident beam from an object through an optical system 10 is controlled by an iris 11 to result in suitable light quantity so that an image is formed on an image pickup element 12. A photoelectric conversion signal according to the formed image is output from the image pickup element 12 and is further converted into a video signal in a signal processing circuit 13 so as to be output.
Meanwhile, the above-described video signal is supplied to an automatic exposure control circuit (hereinafter referred to as xe2x80x9cthe AE circuitxe2x80x9d) 14 which thus generates an iris control signal according to the video signal. A driver 16 receives the iris control signal from the AE circuit 14 via an adder 15 so as to generate a drive signal for driving an ig meter 17, thereby realizing automatic control of the light quantity by use of the iris 11.
The above-described automatic light quantity control is generally based on a principle that the light quantity is controlled according to the integral of different luminance levels of a video signal for one picture. Thus, control is performed in such a way that attention is paid only to the average illuminance in the overall picture. Hence, in performing image pickup of a backlighted object, the incident light to be input into the image pickup element 12 is controlled so that it can be suitable for a very bright background, and accordingly, a main object becomes extremely dark, which thus incurs xe2x80x9cblack compressionxe2x80x9d in the main object.
In order to deal with such a problem, a user for a video camera or the like is required to turn a BLC switch 19 ON to provide an instruction for performing backlight compensation. In the circuit diagram shown in FIG. 22, the BLC switch 19 is turned ON so as to transmit a signal instructing the iris 11 to xe2x80x9copenxe2x80x9d to the driver 16 via the adder 15 from a compensation signal generator 18. The driver 16 drives the ig meter 17 to open so as to increase the amount of incident light to be input into the image pickup element 12, thus performing backlight compensation.
Backlight compensation performed by the conventional method is thus achieved by switching the BLC switch 19 ON, which transforms a resultant image from the state of BLC-OFF (before backlight compensation) shown in FIG. 3 to the state of BLC-ON (after backlight compensation) illustrated in FIG. 4. FIG. 3 clearly shows that in the BLC-OFF state the background is well presented, while the main object (the signpost of Mt. Fuji) is not presented very well, suffering from xe2x80x9cblack compressionxe2x80x9d. In contrast, FIG. 4. shows that in the BLC-ON state the main object is magnificently presented in full expression in all of its gradations.
However, as is seen from FIG. 4, the image pickup apparatus of the above conventional type performs backlight compensation to increase the light quantity by the image pickup element so as to make the main object presentable but also to disadvantageously give rise to xe2x80x9cwhite compressionxe2x80x9d in the background.
In other words, since an image pickup element used in the conventional image pickup apparatus has a restricted dynamic range, the above-described light compensation method for use with such an apparatus presents a problem in that clipping occurs in either of the higher or lower level of illuminance, and it is thus impossible to perform the operation of good image pickup to make both the main object and the background presentable.
This invention aims to solve the above stated problems of the image pickup apparatus of the prior art.
It is, therefore, an object of this invention to provide an image pickup apparatus having a gradation control function which can bring about good exposure for both a main object and its background so as to express a wide range of gradations.
In order to achieve the above object, the present invention provides an image pickup apparatus having a gradation control function, comprising: photoelectric conversion means for photoelectrically converting a light image so as to accumulate light information and to output the light information every unit light accumulation period which forms one picture; signal processing means for processing an output from the photoelectric conversion means to output a video signal; timing means for outputting a plurality of timing signals for setting light information accumulation time every unit light accumulation period which is set in the photoelectric conversion means; and gradation control means for controlling selection of the plurality of timing signals output from the timing means correspondingly with higher luminance and lower luminance regions based on luminance information from the signal processing means, and for inputting the selected timing signal into the photoelectric conversion means.
The present invention also provides an image pickup apparatus having a gradation control function, comprising: photoelectric conversion means for photoelectrically converting a light image so as to accumulate light information and to output the light information every unit light accumulation period which forms one picture; signal processing means for processing an output from the photoelectric conversion means to output a video signal; timing means for outputting a plurality of timing signals for setting light information accumulation time every unit light accumulation period which is set in the photoelectric conversion means; and gradation control means for controlling selection of the plurality of timing signals output from the timing means based on luminance information which is input from the signal processing means every unit light accumulation period which is set in the photoelectric conversion means, and for inputting the selected timing signal into the photoelectric conversion means.
With this arrangement, the image pickup apparatus having a gradation control function according to the present invention performs the following operations. That is, by every unit light accumulation period, light information accumulation time is set longer to achieve exposure suitable for lower luminance object, while it is set shorter to bring about exposure suitable for higher luminance object, based on image information about the respective objects. Thus, the apparatus respectively controls the light information accumulation time so as to accomplish exposure suitable for both higher and lower luminance objects without using mechanical light quantity control means, such as an iris or the like.